Solder bonding method and solder joint

ABSTRACT

A solder bonding method that bonds, using a solder joint, an electrode of a circuit board to an electrode of an electronic component includes: depositing, on the electrode of the circuit board, an Sn—Bi-based solder alloy with a lower melting point than a solder alloy deposited on the electrode of the electronic component; mounting the electronic component on the circuit board such that the Sn—Bi-based solder alloy contacts the solder alloy on the electrode of the electronic component; heating the circuit board to a peak temperature of heating of 150° C. to 180° C.; holding the peak temperature of heating at a holding time of greater than 60 seconds and less than or equal to 150 seconds; and cooling, after the heating and to form the solder joint, the circuit board at a cooling rate greater than or equal to 3° C./sec.

TECHNICAL FIELD

The present invention relates to a solder bonding method for bonding acircuit board and an electronic component by forming a solder join thatexhibits excellent connection reliability and a solder joint.

BACKGROUND

In recent years, with miniaturization and thinning of electronicdevices, high integration of electronic components mounted on electronicdevices has been demanded. High density mounting is necessary for highintegration of electronic components. Flip-chip mounting using BGA, forexample, is an example of such a high density mounting method.

Flip-chip mounting is a method in which electronic components aremounted on a printed circuit board and solder bumps on the printedcircuit board and the BGA are melted and bonded by reflow. Since a largenumber of solder bumps are formed on the printed circuit board, heatingis performed in reflow to such a high temperature that the solder alloyis sufficiently melted to form a solder joint without a defectiveconnection. Therefore, the printed circuit board and the BGA are exposedto such a high temperature. Since the solidus temperature of thecommonly-used SnAgCu solder alloy is about 220° C., the printed circuitboard and the BGA are conventionally exposed to a higher temperaturethan this temperature during reflow.

Under such conditions, thermal warpage of the circuit board may occur.Stress may concentrate on the solder joint during cooling after reflowdue to the difference in coefficient of thermal expansion between theprinted circuit board and the BGA, and the solder joint may break.Moreover, reflow at high temperature may result in high manufacturingcosts.

In view of the above, a proposal has been made on low-temperaturebonding. For example, the Patent Document 1 proposes a solder bondingmethod in which a high-melting-point solder alloy layer is formed on theBGA side, a low-melting-point alloy layer is formed on the printedcircuit board side, these layers are brought into contact with eachother, and thereafter, heating is performed in a temperature range ofthe melting point of the low-melting-point solder alloy or more and lessthan the melting point of the high-melting-point solder alloy. Thismethod allows the high-melting-point solder alloy to remain by heatingin the temperature range and achieves bonding between thelow-melting-point alloy layer and the high-melting-point alloy layer bymelt diffusion, thereby avoiding thermal damage to the circuit board. Inaddition, Patent Document 1 describes an example in which heating ofholding the temperature at 190° C. for 40 seconds is performed. Further,Patent Document 1 shows Sn-58Bi as an example of the low-melting-pointsolder alloy.

PATENT DOCUMENT

Patent Document 1: Japanese Unexamined Patent Application PublicationNo.2000-307228

However, if the heating temperature is 190° C., it is insufficient toprevent thermal damage to the electronic components, and it is necessaryto further lower the heating temperature. However, the inventiondescribed in Patent Document 1 is intended to provide a bonding methodby which a good bonding part can be obtained by a heating process almostthe same as that of tin-lead eutectic solder. Then, the bonding methoddescribed in Patent Document 1 achieves bonding by melt diffusion of thelow-melting-point alloy, which is a second alloy layer. That is, aheating temperature of 190° C. or more is necessary to form a 63Sn-37Pbeutectic alloy layer having a melting point of 183° C. Therefore, if theheating temperature is lowered, it becomes difficult to achieve thebonding by melt diffusion described in Patent Document 1.

Patent Document 1 further describes that heating at a still highertemperature for a longer period of time is required to equalize theelemental distribution in the solder joint. However, heating at a highertemperature requires cooling time. Thus, the intermetallic compoundlayer at the connection interface and the crystal phase of the solderalloy constituting the solder joint grow, and an applied stress isconcentrated at the interface of the intermetallic compound layer andthe interface of the crystal phase. This may result in breakage of thesolder joint from the bonding interface or the region near the bondinginterface where stress is most concentrated at the time of cooling afterheating. It is also conceivable to make the alloy composition of thesolder alloy such that the growth of the intermetallic compound isprevented. However, the composition is limited, and there is a limit toprevention of the growth of the intermetallic compound depending on theheating temperature and time. Especially, Sn-58Bi is used as alow-melting-point solder alloy in Patent Document 1, and Bi in theeutectic portion has properties of being hard and brittle. Thus,breakage of the solder joint is remarkable.

On the other hand, Patent Document 1 describes setting solder bondingconditions to a relatively low temperature and a relatively short timeso as to cause the high-melting-point alloy layer to remain becausesufficient bonding reliability can be obtained even if thehigh-melting-point alloy layer remains. This case makes it difficult forthe structure of the solder joint to become uniform, resulting inbreakage of the solder joint.

SUMMARY

One or more embodiments of the present invention provide a solderbonding method for bonding an electrode of a circuit board and anelectrode of an electronic component by forming a solder joint withexcellent connection reliability while reducing thermal damage to theelectronic component.

According to one or more embodiments, an Sn—Bi-based low-melting-pintsolder alloy may be used as a solder alloy in an electrode on thecircuit board side and lowering the heating temperature from theviewpoint of preventing thermal damage to an electronic component in aconfiguration of forming a high-melting-point solder alloy on theelectronic component side and forming a low-melting-point solder alloyon the printed circuit board side.

Furthermore, according to one or more embodiments, the heating time maybe set to be longer than conventional methods to obtain betterconnection reliability by uniformizing the structure even if the heatingtemperature is lowered. Conventionally, the heating time is usually setto be shortened because heating for a long period of time causes anincrease in manufacturing costs.

Conventionally, from the viewpoint of reducing residual stress caused bythe difference in thermal expansion between the printed circuit boardand the electronic component, the cooling rate after reflow has to bereduced to 2° C./s or less. However, according to one or moreembodiments, as the cooling rate is reduced, the Bi phase of the solderalloy becomes coarse, so the connection reliability of the solder jointis reduced.

According to one or more embodiments of the present invention, byintentionally increasing the cooling rate after reflow while loweringthe reflow temperature compared with the conventional reflow temperatureand heating for a longer period of time than the conventional time,thermal damage to an electronic component is prevented, and the meltdiffusion between the high-melting-point solder alloy on the electroniccomponent side and the low-melting-point solder alloy on the circuitboard side and the melt diffusion between the low-melting-point solderalloy and the electrode are sufficiently performed. Furthermore, thegrowth of the intermetallic compound layer at the bonding interface onthe circuit board side is prevented, and at the same time theSn—Bi-based low-melting-point solder alloy becomes finer, and the stressconcentration at the bonding interface and the crystal interface isprevented, whereby excellent connection reliability can be secured. Inaddition, since the heating temperature is lowered, the residual stressdue to the difference in thermal expansion of the circuit board isreduced, whereby excellent connection reliability of the solder joint ismaintained even when rapid cooling is performed after heating.

The solder bonding method for bonding an electrode of a circuit boardand an electrode of an electronic component by forming a solder joint,according to one or more embodiments of the present invention, includesthe steps of: forming, on the electrode of the circuit board, anSn—Bi-based solder alloy having a lower melting point than a solderalloy formed on the electrode of the electronic component; mounting theelectronic component on the circuit board such that the solder alloyformed on the electrode of the circuit board and the solder alloy formedon the electrode of the electronic component come into contact with eachother; heating the circuit board to a peak temperature of heating of150° C. to 180° C. with a holding time of the peak temperature ofheating of more than 60 seconds and 150 seconds or less; and cooling thecircuit board at a cooling rate after the heating of 3° C./sec or moreto form a solder joint.

The peak temperature of heating may be lower than the melting point ofthe solder alloy formed on the electrode of the electronic component.

In the solder joint, the proportion of the number of Bi phases eachhaving an area of 0.5 μm² or less in the number of Bi phases each havingan area of 5 μm² or less may be 60% or more on average.

The Sn—Bi-based solder alloy may be at least one kind of an Sn—Bi solderalloy, an Sn—Bi—Cu solder alloy, an Sn—Bi—Ni solder alloy, anSn—Bi—Cu—Ni solder alloy, an Sn—Bi—Ag solder alloy, and an Sn—Bi—Sbsolder alloy.

The Bi content in the Sn—Bi-based solder alloy may be 30 to 80 mass %.

The solder alloy formed on the electrode of the electronic component maybe at least one kind of an Sn—Cu solder alloy, an Sn—Ag solder alloy, anSn—Ag—Cu solder alloy, an Sn—Ag—Cu—Ni solder alloy, an Sn—Ag—Cu—Sbsolder alloy, and an Sn—Ag—Cu—Ni—Sb solder alloy.

The temperature difference between the melting point of the solder alloyformed on the electrode of the circuit board and the melting point ofthe solder alloy formed on the electrode of the electronic component maybe 30° C. or more.

The Sn—Bi-based solder alloy may contain 58 mass % of Bi and the balanceof Sn.

In the solder joint according to one or more embodiments of the presentinvention, which bonds an electrode of a circuit board and an electrodeof an electronic component, an Sn—Bi-based solder alloy having a lowermelting point than a solder alloy formed on the electrode of theelectronic component is formed on the electrode of the substrate, and inthe solder joint, the proportion of the number of Bi phases each havingan area of 0.5 μm² or less in the number of Bi phases each having anarea of 5 μm² or less is 60% or more on average.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the Bi content in thesolder joint and the proportion of the number of Bi phases each havingan area of 0.5 μm² or less in the number of Bi phases each having anarea of 5 μm² or less in the case where the temperature profile ischanged after heating.

FIGS. 2A to 2F shows SEM photographs of the breakage surfaces of therespective solder joints obtained by changing the alloy composition ofthe solder alloy formed on the electrode of the circuit board and thetemperature profile at the time of reflow.

Specifically, FIG. 2A is an SEM photograph of the breakage surface ofthe solder joint formed under the conditions of Comparative Example 1.

FIG. 2B is an SEM photograph of the breakage surface of the solder jointformed under the conditions of Example 1 of one or more embodiments ofthe present invention.

FIG. 2C is an SEM photograph of the breakage surface of the solder jointformed under the conditions of Comparative Example 2.

FIG. 2D is an SEM photograph of the breakage surface of the solder jointformed under the conditions of Example 2 of one or more embodiments ofthe present invention.

FIG. 2E is an SEM photograph of the breakage surface of the solder jointformed under the conditions of Comparative Example 3.

FIG. 2F is an SEM photograph of the solder joint formed under theconditions of Example 3 of one or more embodiments of the presentinvention.

FIG. 3 shows a flowchart according to one or more embodiments.

DETAILED DESCRIPTION

The present invention is described in detail below with reference to thefollowing embodiments, which are mere examples, and the presentinvention is not necessarily limited thereby. In the specificationbelow, “%” as to solder alloy composition indicates “mass %” unlessotherwise specifically indicated.

1. Basic Structure of Solder Joint

The solder bonding method according to one or more embodiments of thepresent invention is for bonding an electrode of a circuit board and anelectrode of an electronic component by forming a solder joint.

The solder joint formed by the solder bonding method of one or moreembodiments has a bonding structure of bonding an electrode of a circuitboard and an electrode of an electronic component. The circuit board tobe used in one or more embodiments is a so-called printed circuit board,and a paste has been applied to the electrode of the circuit board.Bumps using solder bolls are formed on the electrode of the electroniccomponent. The solder joint formed by the solder bonding method of oneor more embodiments is formed by bonding the paste applied to theelectrode of the circuit board and the solder bumps formed on theelectrode of the electronic component when the electronic component ismounted on the circuit board.

The circuit board used in one or more embodiments is a conventionalprinted circuit board, and for example, a paper phenol circuit board ora glass-epoxy circuit board can be used. A Cu electrode is mainly usedas the electrode. The electrode may be Ni-plated.

Next, each step will be described in detail.

2. Step of Forming (i.e., Depositing), on the Electrode of the CircuitBoard, an Sn—Bi-Based Solder Alloy having a Lower Melting Point than aSolder Alloy formed (i.e., Deposited) on the Electrode of the ElectronicComponent

(1) Method for Forming Solder Alloy

In the solder bonding method of one or more embodiments, first, a solderalloy is formed on each of an electrode of a circuit board and anelectrode of an electronic component. A high-melting-point solder alloythat does not melt at a reflow temperature is formed on the electrode ofthe electronic component, and an Sn—Bi-based low-melting-point solderalloy that melts at a reflow temperature is formed on the electrode ofthe circuit board.

Specifically, for example, a mask is placed on the electrode of thecircuit board, and a paste on the mask is coated to the electrode of thecircuit board through an opening of the mask while moving a squeegee.Alternatively, the paste may be applied to the electrode by a dischargemethod or pre-coating. The coating thickness is not particularly limitedand may be 0.05 to 0.2 mm The components of the paste other than asolder alloy powder may be those conventionally used and are notparticularly limited. The flux used in the solder paste may be either awater-soluble flux or a water-insoluble flux. A rosin-based flux, whichis a rosin-based water-insoluble flux, is generally used.

On the electrode of the electronic component, bumps are formed bymounting solder balls having a diameter of 100 to 1000 μm depending on adiameter of the electrode.

(2) Alloy Composition of Solder Alloy Formed on Electrode of CircuitBoard

An Sn—Bi-based solder alloy having a lower melting point than a solderalloy formed on the electrode of the electronic component is formed onthe electrode of the circuit board to be used in one or moreembodiments. Such low-melting-point solder alloy is necessary forlow-temperature bonding to prevent thermal damage to the circuit board.Accordingly, the solder alloy used on the circuit board side of one ormore embodiment may have a low melting point (liquidus temperature) of150° C. or less. Such an Sn—Bi-based solder alloy can be at least onekind of Sn—Bi solder alloys, Sn—Bi—Cu solder alloys, Sn—Bi—Ni solderalloys, Sn—Bi—Cu—Ni solder alloys, Sn—Bi—Ag solder alloys, and anSn—Bi—Sb solder alloys.

When Cu or Ni is added to the Sn—Bi solder alloy, the Cu content may be0.1% to 1.0%, and the Ni content may be 0.01% to 0.1%. These Sn—Bi—Cusolder alloy, Sn—Bi—Ni solder alloy, and Sn—Bi—Cu—Ni solder alloy canprevent the growth of intermetallic compound at the bonding interface ofthe solder joint, so that excellent connection reliability can bemaintained even when heating is performed for a long period of time asin one or more embodiments. The solder alloy may further contain Ag orSb in a range in which the melting point of the solder alloy becomes150° C. or less.

In addition, the Bi content may be 30% to 80% in alloy composition ofeach of these alloys. When the Bi content is in the above-describedrange, the melting point (solidus temperature) is constant at 138° C.Therefore, when an alloy having such Bi content is used for solder bumpson the circuit board, the solder alloy formed on the electrode of thecircuit board starts melting when heated, and the bumps of theelectronic component can form a solder joint while pressing the bumps ofthe circuit board by the self-weight of the electronic component. Inaddition, the Bi content may be 35% to 70% or 53% to 61% from theviewpoint of shifting the liquidus temperature to a low temperature tolower the heating temperature and further reducing the thermal damage tothe electronic component and the circuit board.

The Sn—Bi-based solder alloy is particularly preferably an alloy having58% of Bi and the balance of Sn as eutectic composition. Since thisalloy composition is eutectic composition, it becomes a liquid phase ata temperature of 138° C. or higher, and thus a solder joint can beeasily formed at a low temperature.

(3) Alloy Composition of Solder Alloy Formed on Electrode of ElectronicComponent

The solder alloy formed on the electrode of the electronic component ispreferably, for example, at least one kind of an Sn—Cu solder alloy, anSn—Ag solder alloy, an Sn—Ag—Su solder alloy, an Sn—Ag—Cu—Ni solderalloy, an Sn—Ag—Cu—Sb solder alloy, and an Sn—Ag—Cu—Ni—Sb solder alloy.

These solder alloys preferably have such melting points that they do notmelt at the time of reflow and may have melting points (solidustemperatures) of 200° C. or more.

(4) Temperature Difference in Melting Point Between Solder Alloys

The temperature difference between the melting point (liquidustemperature) of the solder alloy formed on the electrode of the circuitboard and the melting point (solidus temperature) of the solder alloyformed on the electrode of the electronic component may be 30° C. ormore. When the temperature difference is 30° C. or more, the solderalloy formed on the electrode of the electronic component does not melteven considering the error in temperature control. The temperaturedifference in melting point is more preferably 40° C. or more, yet morepreferably 50° C. or more. For example, the liquidus temperature andsolidus temperature can be measured using DSC.

3. Step of Mounting the Electronic Component on the Circuit Board suchthat the Solder Alloy Formed on the Electrode of the Circuit Board andthe Solder Alloy Formed on the Electrode of the Electronic Componentcome into Contact with Each Other.

Next, the electronic component is mounted on the circuit board such thatthe electrode of the circuit board and the electrode of the electroniccomponent come into contact with each other. The electronic componentwith bumps formed thereon is supplied by a feeder, and the electroniccomponent is mounted on the circuit board by a component mountingapparatus. Examples of the system of the component mounting apparatusinclude a one-by-one system, an in-line system, and a multisystem.

4. Step of Heating the Circuit Board to a Peak Temperature of Heating of150° C. to 180° C. with a Holding Time of the Peak Temperature ofHeating of More than 60 Seconds and 150 Seconds or Less

(1) Peak Temperature of Heating

The circuit board on which the electronic component is mounted isintroduced into a reflow furnace, and the circuit board is heated at apeak temperature of heating of 150° C. to 180° C. during reflow. Thistemperature range may be lower than the melting point of the solderalloy formed on the electrode of the electronic component. That is, thistemperature range may be a temperature range in which the solder alloyformed on the electrode of the circuit board melts, and the solder alloyformed on the electrode of the electronic component does not melt. Inthis temperature range, thermal damage to the circuit board and theelectronic component can be prevented. In addition, since the heatingtemperature is kept low, the influence of the difference in coefficientof thermal expansion among the circuit board, the electronic component,and the solder alloy is reduced, whereby stress concentration at thesolder joint during rapid cooling can be prevented.

If the peak temperature of heating is less than 150° C., the Bi phase issegregated, and a microstructure cannot be obtained. In addition, themelt diffusion is insufficient, and therefore, the bonding between theelectrode and the solder alloy or the bonding between thehigh-melting-point solder alloy and the low-melting-point solder alloymay not be completed, resulting in breakage of the solder joint in somecases. On the other hand, if the peak temperature of heating exceeds180° C., thermal damage to the electronic component and the circuitboard may occur. Therefore, the peak temperature of heating ispreferably 165° C. to 180° C.

(2) Holding Time

The holding time of the peak temperature of heating is more than 60seconds and 150 seconds or less. Conventionally, the holding time issuppressed to at most about 40 seconds from the viewpoint of shorteningthe manufacturing time and reducing the costs. However, in one or moreembodiments, in order to improve connection reliability of the solderjoint, a sufficient melting time is ensured by heating for a long periodof time, which has been avoided conventionally. Thus, the structure isuniformized, and the melt diffusion is sufficiently performed. In thesolder bonding method of one or more embodiments, the peak temperatureof heating is kept low as mentioned above. Thus, the growth of theintermetallic compound layer at the bonding interface can be prevented,and the crystal grain size of the solder alloy can be reduced even ifthe holding time is lengthened. As a result, excellent connectionreliability can be obtained.

If the holding time is 60 seconds or less, the Bi phase segregates atthe time of solidification, and the melt diffusion is insufficient, sothat the bonding is not completed. Therefore, if the cooling rate ishigh, the solder joint breaks due to the difference in coefficient ofthermal expansion among the circuit board, the electronic component, andthe solder alloy, as mentioned below. On the other hand, if the holdingtime exceeds 150 seconds, the manufacturing time becomes too long, whichis not preferable. The holding time of the peak temperature of heatingis more preferably 90 to 120 seconds from the viewpoint of reliablymelting and diffusing and not reducing the productivity.

In order to remove the solvent in the paste, preheating may be performedat a temperature in the range of 50° C. to 100° C. before introducingthe circuit board into the reflow furnace.

5. Step of Cooling the Circuit Board at a Cooling Rate After the Heatingof 3° C./Sec or More to Form a Solder Joint.

The circuit board is cooled from the heating temperature to roomtemperature at a cooling rate after the heating in the range of 3°C./sec or more. Conventionally, cooling after reflow is performed byair-cooling, and this cooling is performed at a cooling rate ofapproximately 1° C./sec. This is to avoid stress concentration at thesolder joint due to the difference in coefficient of thermal expansionamong the circuit board, the electronic component, and the solder alloy.However, in one or more embodiments, by separately providing a coolingmeans in the reflow furnace to increase the cooling rate of the circuitboard, growth of the intermetallic compound layer at the bondinginterface is prevented, and at the same time the Bi phase in theSn—Bi-based low-melting-point solder alloy becomes fine, and stressconcentration at the bonding interface and the crystal interface isprevented, whereby excellent connection reliability can be ensured. Inaddition, since the heating temperature is lowered, residual stress dueto the difference in thermal expansion of the circuit board is reduced,whereby excellent connection reliability of the solder joint can bemaintained even when rapid cooling is performed after heating.

When the cooling rate is less than 3° C./sec, it takes time to cool, sothat the Bi phase becomes coarse as the intermetallic compound layergrows. On the other hand, the upper limit of the cooling rate ispreferably 7° C./sec or less, more preferably 5° C./sec or less, yetmore preferably 4° C./sec or less from the viewpoint of coolingequipment.

The means for increasing the cooling rate is not particularly limited,and the circuit board may be cooled by cold air using a compressor orthe like, or may be cooled by pressing a cooling medium against thecircuit board. However, cooling using a compressor is preferable fromthe viewpoint of ensuring a stable cooling rate.

In the solder bonding method according to one or more embodiments,bonding is performed at a lower temperature than that in theconventional method, whereby the damage to the circuit board and theelectronic component can be prevented. Further, the damage to theheating element of the reflow furnace can also be prevented, whereby thecosts can be reduced.

6. Solder Joint

In observation of the cross section of the solder joint formed by thesolder bonding method according to one or more embodiments, theproportion of the number of Bi phases each having an area of 0.5 μm² orless in the number of Bi phases each having an area of 5 μm² or less inany region on the electrode side of the circuit board may be 60% or moreon average. In one or more embodiments, excellent connection reliabilitythat has not been achieved conventionally can be achieved by focusing onthe structure of the low-melting-point solder alloy and controlling thestructure of the solder alloy by setting the heating conditions and thecooling conditions to predetermined ranges as mentioned above. In orderto achieve this, it is necessary to focus on the area of the Bi phase,which is essential for the low-melting-point solder alloy. That is, ifthe number of fine Bi phases each having a small area is larger than thenumber of coarse Bi phases each having a large area in observation ofthe cross section of the solder joint, a large number of fine Bi phasesare present, and therefore, the microstructure can be obtained.

Therefore, in one or more embodiments in which the fine Bi phase isachieved, even when a large stress is applied, the stress is dispersed.Thus, even if a circuit board and an electronic component havingcoefficients of thermal expansion largely different from each other areused, the stress can be relieved to such an extent that the circuitboard and the electronic component do not break.

The solder joint capable of exhibiting such effect is required to havean alloy structure having a large number of Bi phases each having anarea of 0.5 μm² or less. In order to make the above effect moresufficiently exhibited, the proportion of the number of Bi phases eachhaving an area of 0.5 μm² or less in the number of Bi phases each havingan area of 5 μm² or less may be 60% or more on average or 65% or more onaverage.

The solder joint according to one or more embodiments may have anintermetallic compound layer of 3 to 5 μm in the vicinity of the bondinginterface with the electrode.

EXAMPLES

1. Production of Solder Joint

Table 1 summarizes alloy composition of a solder alloy formed on a Cuelectrode of a circuit board, heating conditions, and coolingconditions. The circuit board used in the present examples was an FR-4glass-epoxy circuit board with an electrode diameter of 325 μm, a sidelength of 105 mm, and a plate thickness of 0.8 mm On this circuit board,an electronic component (BGA) with a Cu electrode, having an electrodediameter of 450 μm and a side length of 20 mm, was mounted. The alloycomposition of the solder alloy formed on the electrode of BGA (referredto as “Device” in Table 1) was SAC405 (Sn-4Ag-0.5 Cu (Ag: 4%, Cu: 0.5%,Sn: the balance), solidus temperature: 217° C.) as summarized in Table1, and bumps were formed on the electrode using solder balls with adiameter of 290 μm. The alloy composition of the solder alloy formed onthe electrode of the circuit board is as summarized in Table 1. In thealloy composition described in the column “Alloy Composition” in Table1, the numerical value described right before the element represents thecontent (mass %), and the balance other than Bi, Cu, and Ni representsSn. It was confirmed by DSC that the liquidus temperature of each solderalloy on the circuit board side was lower than the solidus temperatureon the BGA side. The liquidus temperature of Sn-58Bi was 141° C., theliquidus temperature of Sn-35Bi-0.5Cu-0.03Ni was 184° C., and theliquidus temperature of Sn-70Bi-0.5Cu-0.03Ni was 180° C. The solidustemperature and the liquidus temperature were determined from the DSCcurve obtained by increasing the temperature at 5° C./min in theatmosphere using a DSC (Model No. Q2000) manufactured by TA InstrumentsJapan, Inc.

A solder paste containing solder alloy powder of this alloy compositionwas applied to the electrode of the circuit board to have a thickness of100 μm. Then, the circuit board was heated and cooled under theconditions summarized in Table 1 to form a solder joint having athickness of 200 μm. The heating temperature and the cooling rate weremeasured by attaching a thermocouple to the circuit board.

As a method of cooling the solder alloy, in the examples and ComparativeExamples 5 and 6, the surface of the circuit board after heating,opposite to the surface on which the BGA was mounted was rapidly cooledby pressing it against a cooling agent, and in Comparative Examples 1 to4, the circuit board was cooled in the atmosphere. The cooling rate wasdetermined by measuring the temperature and time until the circuit boardwas cooled to room temperature by bringing the thermocouple into contactwith the circuit board.

2. Observation of Breakage Surface of Solder Joint

An SEM photograph of the breakage surface of the formed solder joint wastaken with a scanning electron microscope (JSM-5600LV manufactured byNippon Electronics Co., Ltd.) at 1000× magnification with theobservation mode as the BEI. Three regions of 128 μm×96 μm werearbitrarily selected from the region near the bonding interface of thesolder joint, and the area of the Bi phase at each of three locationswas determined. The area of the Bi phase was determined by assumingthat, in the SEM photograph, a portion of 1 dot had 0.04 μm², a portionof up to 2 dots was a noise, and a white portion of three dots having0.12 μm² or more was determined as the Bi phase. Among the determinedareas of the Bi phases, the one having the area of 0.5 μm² or less wasdetermined as the fine Bi phase. The proportion of the number of Biphases each having an area of 0.5 μm² or less in the number of Bi phaseseach having an area of 5 μm² or less in each region was measured, andeach average was determined.

3. Connection Reliability Test

A strain gauge was attached to the four corners of the BGA using themounting circuit board on which the solder joint was formed as describedin the item 1 above, and the bending test was repeatedly performed whilemonitoring the strain amount at all times. In order to evaluate theresults of the bending test, the bonding impedance (Ω) between the BGAand the circuit board was measured, and the number of cycles at whichthe impedance increased by 10% or more from the initial value wasmeasured.

The number of bending cycles at a strain amount of 1000 μ Strain of100000 was set to the maximum number of bending cycles for the mountingcircuit board on which the used electronic component (BGA) was mounted.If the maximum number of bending cycles was achieved, it was determinedthat there was no problem even if the electronic component was used forthe products, so the solder joint was evaluated as “∘”. If the number ofbending cycles was less than 100000, the solder joint was evaluated as“×”. The bending test was conducted based on “IPC/JEDEC-9707 SphericalBend Test Method for board level Interconnects”.

The results of the measurements are shown in Table 1.

TABLE 1 Alloy Composition Peak Degree of fine geometry Device PasteTemp. Keep Cooling Bump 1 Bump 2 Bump 3 Ave. Bending test result Ex. 1SAC405 Sn—35Bi—0.5Cu—0.03Ni 180 120 4 64.5 61.0 59.7 62 ∘ Ex. 2 SAC405Sn—58Bi 180 120 4 65.4 58.1 55.3 60 ∘ Ex. 3 SAC405 Sn—70Bi—0.5Cu—0.03Ni180 120 4 65.8 58.4 57.9 61 ∘ Ex. 4 SAC405 Sn—58Bi 165 120 4 62.7 64.861.4 63 ∘ Ex. 5 SAC405 Sn—58Bi 150 120 4 64.6 66.4 60.9 64 ∘ Ex. 6SAC405 Sn—58Bi 180 120 3 60.5 67.8 67.6 65 ∘ Ex. 7 SAC405 Sn—58Bi 180120 7 73.8 73.5 71.5 73 ∘ Ex. 8 SAC405 Sn—58Bi 180 150 4 72.3 73.3 66.771 ∘ Comp. Ex. 1 SAC405 Sn—35Bi—0.5Cu—0.03Ni 180 30 1 52.7 50.1 45.8 50x Comp. Ex. 2 SAC405 Sn—58Bi 180 30 1 49.2 46.6 34.2 43 x Comp. Ex. 3SAC405 Sn—70Bi—0.5Cu—0.03Ni 180 30 1 53.7 50.2 43.6 49 x Comp. Ex. 4SAC405 Sn—58Bi 180 120 1 58.1 46.3 56.9 54 x Comp. Ex. 5 SAC405 Sn—58Bi180 30 4 47.4 43.6 47.0 46 x Comp. Ex. 6 SAC405 Sn—58Bi 180 60 4 58.060.2 59.7 59 x

As is evident from Table 1, the solder joints formed by the reflowprofiles of the examples showed the results of the proportion of thenumber of Bi phases each having an area of 0.5 μm² or less in the numberof Bi phases each having an area of 5 μm² or less of 60% or more onaverage. This demonstrates that sufficiently fine Bi phases wereobtained by rapid cooling after heating. The results of the repeatedbending test mentioned in the item 3 above showed that 100000 cycles,which is the maximum number of bending cycles were achieved.

In contrast, the solder joints formed under the conditions of thecomparative examples only showed the results of the proportion of thenumber of Bi phases each having an area of 0.5 μm² or less in the numberof Bi phases each having an area of 5 μm² or less of below 60% onaverage. In the connection reliability test mentioned in the item 3above, none of the compositions achieved 100000 cycles because theimpedance increased by 10% or more at the time when 100000 cycles werenot achieved.

In Comparative Examples 1 to 4, the number of bending cycles was smallbecause the cooling rate was slow, and sufficiently fine Bi phase couldnot be obtained. In Comparative Examples 5 and 6, the Bi phase wassegregated to have low degree of fine geometry because the holding timeof heating was short, and the number of bending cycles was small.

Some of the results of Table 1 are extracted and shown in FIG. 1. FIG. 1is a graph showing the relationship between the Bi content in the solderjoint and the proportion of the number of Bi phases each having an areaof 0.5 μm² or less in the number of Bi phases each having an area of 5μm² or less in the case where the temperature profile is changed afterheating. In FIG. 1, the vertical axis indicates the proportion of thenumber of Bi phases each having an area of 0.5 μm² or less in the numberof Bi phases each having an area of 5 μm² or less, and the horizontalaxis indicates the Bi content in the solder alloy formed on theelectrode of the circuit board. As the specific alloy composition, the“35Bi” indicates Sn-35Bi-0.5Cu-0.03Ni (the composition used in Example 1and Comparative Example 1), the “58Bi” indicates Sn-58Bi (thecomposition used in Example 2 and Comparative Example 2), and the “70Bi”indicates Sn-70Bi-0.5Cu-0.03Ni (the composition used in Example 3 andComparative Example 3). The “sight1”, the “sight2”, and the “sight3”represent the respective three regions selected from the breakagesurface of the solder joint, and the “Ave.” represents the average ofvalues in each alloy composition. As shown in FIG. 1, by optimizing thereflow profile, the proportion of the average of the number of Bi phaseseach having an area of 0.5 μm² or less in the average of the number ofBi phases each having an area of 5 μm² or less became high in thepresent examples, regardless of the solder alloy composition.

Photographs of the breakage surfaces of the solder joints formed in theabove-described manner are shown. FIGS. 2A to 2F shows SEM photographsof the breakage surfaces of the respective solder joints obtained bychanging the alloy composition of the solder alloy formed on theelectrode of the circuit board and the temperature profile at the timeof reflow. FIG. 2A is an SEM photograph of the breakage surface of thesolder joint formed under the conditions of Comparative Example 1. FIG.2B is an SEM photograph of the breakage surface of the solder jointformed under the conditions of Example 1. FIG. 2C is an SEM photographof the breakage surface of the solder joint formed under the conditionsof Comparative Example 2. FIG. 2D is an SEM photograph of the breakagesurface of the solder joint formed under the conditions of Example 2.FIG. 2E is an SEM photograph of the breakage surface of the solder jointformed under the conditions of Comparative Example 3. FIG. 2F is an SEMphotograph of the solder joint formed under the conditions of Example 3.

In FIGS. 2A to 2F, the white part is the Bi phase. As shown in FIGS. 2B,2D and 2F, in Examples 1 to 3 in which the reflow profiles wereoptimized, the fine Bi phases could be observed. In contrast, as shownin FIGS. 2A, 2C and 2E, in Comparative Examples 1 to 3, each of Biphases had a large area and was coarse as a whole compared with Examples1 to 3. The same results were obtained in other examples and comparativeexamples. In Examples 1 to 3, it is considered that the resultsexceeding 100000 cycles in the repeated bending test were obtained bymaking the structure of the solder alloy constituting the solder jointfine and relaxing the stress.

FIG. 3 shows a flowchart according to one or more embodiments.Specifically, the flowchart depicts the method for forming a solderjoint between an electrode of a circuit board and an electrode of anelectronic component discussed above. The scope of the invention shouldnot be considered limited to the specific arrangement of steps shown inFIG. 3.

In STEP 305, as discussed above, a Sn—Bi based solder alloy is depositedon an electrode of a circuit board and a solder alloy is deposited on anelectrode of an electronic component. In one or more embodiments, themelting point of the Sn—Bi-based solder alloy is lower than the meltingpoint of the solder alloy deposited on the electrode of the electroniccomponent.

In STEP 310, as discussed above, the electronic component is mountedonto the circuit board in order for the Sn—Bi-based solder alloy tocontact the solder alloy deposited on the electronic component.

In STEP 315, as discussed above, the circuit board is heated to a peaktemperature of heating and the peak temperature of heating is held at apredetermined holding time.

In STEP 320, as discussed above, the circuit board is cooled, after theheating, to a predetermined cooling rate in order to form the solderjoint between the electrode of the circuit board and the electrode ofthe electronic component.

As described above, by forming the solder joint by the solder bondingmethod according to one or more embodiments, thermal damage to thecircuit board and the electronic component can be reduced, and excellentconnection reliability can be exhibited.

Although the disclosure has been described with respect to only alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that various other embodiments maybe devised without departing from the scope of the present invention.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A solder bonding method that bonds, using asolder joint, an electrode of a circuit board to an electrode of anelectronic component, the method comprising: depositing, on theelectrode of the circuit board, an Sn—Bi-based solder alloy with a lowermelting point than a solder alloy deposited on the electrode of theelectronic component; mounting the electronic component on the circuitboard such that the Sn—Bi-based solder alloy contacts the solder alloyon the electrode of the electronic component; heating the circuit boardto a peak temperature of heating of 150° C. to 180° C.; holding the peaktemperature of heating at a holding time of greater than 60 seconds andless than or equal to 150 seconds; and cooling, after the heating and toform the solder joint, the circuit board at a cooling rate greater thanor equal to 3° C./sec.
 2. The solder bonding method according to claim1, wherein, in the solder joint, the peak temperature of heating islower than a melting point of the solder alloy deposited on theelectrode of the electronic component.
 3. The solder bonding methodaccording to claim 1, wherein a proportion of Bi phases each having anarea of less than or equal to 0.5 μm² among Bi phases each having anarea of less than or equal to 5 μm² is greater than or equal to 60%. 4.The solder bonding method according to claim 1, wherein the Sn—Bi-basedsolder alloy is at least one of an Sn—Bi solder alloy, an Sn—Bi—Cusolder alloy, an Sn—Bi—Ni solder alloy, an Sn—Bi—Cu—Ni solder alloy, anSn—Bi—Ag solder alloy, or an Sn—Bi—Sb solder alloy.
 5. The solderbonding method according to claim 1, wherein a Bi content in theSn—Bi-based solder alloy is 30 to 80 mass %.
 6. The solder bondingmethod according to claim 1, wherein the solder alloy deposited on theelectrode of the electronic component is at least one of an Sn—Cu solderalloy, an Sn—Ag solder alloy, an Sn—Ag—Cu solder alloy, an Sn—Ag—Cu—Nisolder alloy, an Sn—Ag—Cu—Sb solder alloy, or an Sn—Ag—Cu—Ni—Sb solderalloy.
 7. The solder bonding method according to claim 1, wherein atemperature difference between a melting point of the Sn—Bi-based solderalloy and a melting point of a solder alloy deposited on the electrodeof the electronic component is greater than or equal to 30° C.
 8. Thesolder bonding method according to claim 1, wherein the Sn—Bi-basedsolder alloy contains 58 mass % of Bi and a balance of Sn.
 9. A solderjoint for bonding an electrode of a circuit board to an electrode of anelectronic component, the solder joint comprising: an Sn—Bi-based solderdeposited on the electrode of the circuit board; and a solder alloydeposited on the electrode of the electronic component, wherein theSn—Bi-based solder alloy has a lower melting point than the solder alloydeposited on the electrode of the electronic component, and in thesolder joint, a proportion of Bi phases each having an area of less thanor equal to 0.5 μm² among Bi phases each having an area of less than orequal to 5 μm² is greater than or equal to 60%.